The present invention provides methods and apparatus for repairing inoperative pixels in a display. In particular, the present invention provides methods and apparatus for improving the effective yield rates of displays, such as liquid crystal micro-displays, by disconnecting inoperative pixels from their defective drive circuitry and tying such pixels to the working drive circuit of a nearby pixel.
Manufacturing of displays, such as liquid crystal (LC) micro-displays, with very large numbers of pixels is hampered by low yield rates due to the large area of semiconductor material (e.g., Complementary Metal Oxide Semiconductor (CMOS)) required for each device. Nevertheless, displays can actually tolerate a certain level of various types of defects and still be considered acceptable. Additionally, there are various techniques that have been developed and successfully applied to other display technologies that can reduce the visibility of some types of defects and dramatically increase the yield of acceptable displays.
For example, on a device size suitable to hold 8 million pixels (approximately 32×58 mm), without any repair strategy, the yield of “perfect” display devices is estimated at less than 2% (assumed defect rate of 0.2/cm2).
Defects in the microelectronic circuitry can cause a variety of types of malfunctions in the resulting display, such as:
Stuck-OnThese pixels are always on. In a three-chip RGBPixel(red, green, blue) system, they would be visibleas a constant red, green, or blue dot in thedisplay. Such stuck-on pixels are much more visiblein dark areas of the display than bright areas.Stuck-OffThese pixels are always off. In a three-chip RGBPixelsystem, they would be visible as a constant cyan,magenta, or yellow dot in the display. Such stuck-off pixels are much more visible in the brightareas of the display than the dark areas.Stuck-Some pixels can get stuck in an intermediate state.IntermediateDepending on the intended color of the pixel andthe surrounding area of the display, they willsometimes appear as red, green, or blue, and atother times will appear as the complement of thecolor channel affected.PartialSome pixels may have a portion of their circuitryResponseaffected such that they partially respond to theintended color. Perhaps they are of reducedintensity or contrast, but they still track theintended value to some degree.DefectiveA significant portion of the circuitry in theClumpvisible area may be shared between clumps of pixels.A defect in this area may effect the entire clumpof pixels and may take any of the above forms.DeadDefects in signals generated by the drive circuitryColumn/Rowwhich are fed to every pixel in a column or rowwill manifest themselves in the entire column orrow. The manifestation may take a variety of forms.
Although a display having any of the foregoing defects may still be useable, all such defects are significant enough that once noticed, the user will be unhappy. The only exception being perhaps a few isolated stuck-off pixels in the display used for the blue channel, due to the low contrast sensitivity of human vision to blue light.
A previously known method for dealing with such defects is to provide redundant circuitry in the display that can be selected to drive a pixel or column of pixels in the event of a defect in the primary circuitry. As it is already difficult to fit all required circuitry in the space available in such micro-displays, the inclusion of redundant circuitry becomes problematic.
It would be advantageous to provide a method of repairing inoperative pixels in a display without requiring redundant circuitry in order to increase the yield rate of such displays.
The methods and apparatus of the present invention provide the aforementioned and other advantages.